1. Field of the Invention
The present invention relates to lasers and optical compensation of laser radiation. More specifically, this invention is concerned with means and methods for creating a temperature gradient within a body of optical material for establishing radially dependent optical path length and polarization fields which can be exploited to compensate for anomalies in laser rods.
2. Background Information
One of the most burdensome problems which confronts designers of communication and measuring systems which employ lasers is the phenomenon of beam divergence. Although lasers produce coherent light which is commonly perceived to be constituted of a multitude of perfectly parallel rays of electromagnetic radiation, these rays always spread to some extent. Excessive divergence of the beam occurs if the optical components comprising the resonator cause optical distortions. Various laser amplification media and systemic constraints cause distortion in laser rods. Typical repetitively pumped solid state laser rods, for example, tend to produce output radiation which diverges in proportion to the pumping power of the laser. An optical device which causes radiation to diverge exhibits the most significant property of a convex or positive lens, as depicted in FIG. 3a.
If an incident beam of light composed of collimated or parallel rays is directed toward a positive lens of uniform and isotropic temperature and is aligned parallel to the optical axis of the lens, then the direction of each ray that issues from the exit side of the lens will depend upon the angle of its incidence relative to the surface normal on the face of the lens and upon the optical path length which the ray must traverse in order to travel across the lens. Each ray will be bent by an amount which is dependent upon its angle of incidence relative to the surface normal and its path length which, in turn, depends upon the index of refraction of the optical material from which the lens is made and the thickness of the lens which the ray traverses.
In the case of a repetitively pumped laser rod, the rod will be heated by an amount proportional to the input power the rod absorbs from the excitation mechanism. The heat in the rod is dissipated at its exterior surface. Gradually, perhaps over a period of ten seconds, the rod is transformed into a positive lens by this heating and cooling. This transformation takes place because the rod is not uniformly and isotropically homogeneous with respect to temperature. The temperature differences along any radius of the rod set up regions of different indices of refraction. A radial temperature gradient is created within the rod, and, since the refractive index is temperature dependent, a radial refractive index gradient is also established within the rod. This effectively creates a lens having a distribution of radially dependent optical path lengths which mimics that of a physically convex lens which is depicted in FIG. 3a. The power of this transitory positive lens is proportional to the extent to which it is heated because the temperature determines the spatial variation of refraction within it. The temperature gradient generated in the rod is proportional to the heat flow and the refractive index parallels the temperature gradient. This dynamic lensing action of the rod, brought about by a temperature gradient, is responsible for the unwanted beam divergence.
When the output beam of a laser diverges in this way, the coherent radiation loses much of its effectiveness, since the energy delivered by the beam to a distant point depends on how much of the beam is concentrated on a targeted spot of limited area. Energy which is spread out over a much larger area than the cross-section of the original beam is less concentrated, and therefore less useful, in communications and measurement applications.
Various complex mechanical systems have been employed to in previous attempts to solve this problem of rod lensing. One such system is a zoom lens which consists of a pair of short focal length lenses of opposite optical power. The spacing between the lenses can be adjusted to control the net optical power resulting from the use of the pair of lenses. This arrangement maintains a constant beam divergence, but the exorbitant cost of the necessary precision lenses, mechanical races, direct current motor, and complicated control electronics makes this solution unattractive.
Simple static concave negative lenses have been used in the laser resonator cavity to compensate for the dynamic lensing action of the rod. The constant action of the static negative lens, however, proves detrimental to the operation of the laser until the time when the laser rod has been heated and has attained the exactly equivalent but opposite optical power for which the simple fixed negative lens is designed to compensate. A further complication results from the use of variable repetition rates of the pulsed laser. One static concave negative lens can not cope with variable rates, since the best achievable compensation is for one particular, specified magnitude or extent of dynamic lensing caused by the heated laser rod.
Another problem concerning the performance of a solid state laser is the optical aberration of laser rod due to thermal birefringence in the laser rod which causes depolarization of the laser beam and reduces efficiency in polarized lasers. When a solid state rod is pumped with excitation radiation, a great portion of this stimulation energy is converted to heat in the rod. One of the effects of this rod heating is the depolarization of the laser beam by birefringence. Beam depolarization can reduce the potency of laser output and therefore poses a serious problem when lasers are used in situations which require high power output.
In a solid state rod, heat resulting from flashlamp excitation causes physical deformation of the rod. Since the rod material expands with temperature, a radial stress gradient is formed which produces radially dependent birefringence in the rod. Birefringence, also known as double refraction, is an optical phenomenon in which a material exhibits a different index of refraction for each of two polarization directions defined by the material. This double refraction is illustrated by the action of a birefringent crystal in FIGS. 3c-3g. In the course of the passage of light through birefringent medium resolves the polarized beam into two component beams. Each beam is polarized along one of the unique directions, so that the beams traverse the material at different speeds. When the two beams recombine after leaving the material, they are no longer in phase with one another and the polarization state is changed.
FIGS. 3c-g schematically illustrate this birefringence phenomenon. In FIG. 3c, vector I represents the polarization direction of a beam of incident light. When this beam I passes through a birefringent material, as is shown in cross-section in FIG. 3d, the vector I is resolved into two components travelling at different speeds, S, shown in solid line, and F, shown in dashed line. Components S and F, which represent high and low refractive index polarizations, respectively, are shown after exiting the crystal in FIG. 3e. The S component now lags the F component as a result of the birefringent action (FIG. 3f). When added vectorially, S and F combine to form the resultant vector R which has a new polarization direction (as shown in FIG. 3g).
The heat in the laser rod creates depolarizing regions within it which vary in efficacy in accordance with each internal region's distance from the central axis of the rod. The different regions cause the rod as a whole to become birefringent because of variations in the indices of refraction of the differently stressed portions of the rod. As was the case with the limitations of compensation of beam divergence by the action of a single, simple negative lens placed within the resonator cavity, such a lone optical element is equally ineffective in correcting for depolarization caused by laser rod thermal stress birefringence.
Previously known methods of compensating for rod birefringence include the use of a polarization rotator used between a pair of rods operated at the same output power level. A lensing system is employed to align the beam so that the rods compensate for each other's birefringence. This system, however, requires the use of two rods in addition to pressure vessels which are extremely expensive and difficult to maintain in proper mechanical alignment.
A number of inventions noted in the disclosure statement filed in connection with this application employ devices which modify a beam of radiation using an externally controlled optical medium in order to alter various properties of the beam. U.S. Pat. No. 3,736,046--Zook, discloses apparatus which adjusts the wavefront shape of a light beam using beam-addressed optical memory means controlled by a varying electric field. Mitchell et al. describe an optical device which exhibits temperature-dependent optical absorption properties in U.S. Pat. No. 3,790,250. An apparatus devised by Drake in U.S. Pat. No. 3,945,715 employs an electro-optical transducer in a large scale data storage system. Hon et al. (U.S. Pat. No. 4,019,159) use a feedback arrangement with a crystal of electro-optic material mounted in an oven to control the temperature and tune the electric field of a frequency doubling crystal. In U.S. Pat. No. 4,117,399, Ono et al. explain a method and apparatus for measuring electric current or voltage which utilizes an optical converter which includes a source of laser light, a polarizer, and a Faraday rotator. Huignard et al. exploit the Kerr Effect by subjecting two astigmatic electro-optical elements to varying electric fields in U.S. Pat. No. 4,124,273 in order to focus an incident beam of energy on an object which is changing its position rapidly. A thermal-optical converter comprising a closed vessel containing an aqueous solution is disclosed by Yamada et al., in U.S. Pat. No. 4,169,661.
None of the preceding inventions solve the problem of the deleterious effects of beam divergence or depolarization produced by thermally induced birefringence in a solid state rod laser. Three of the inventions cited in the disclosure statement are more directly concerned with the problem of using an externally controlled optical medium to accomplish some compensation of aberrations in a beam of radiation. Kumada discloses apparatus comprising an electro-optical crystal used in conjunction with a power source which is used to impose a voltage that regulates the transmission of incident light through the crystal. This optical switch, which is described in U.S. Pat. No. 3,838,906, restricts light transmission by altering the birefringent characteristics of the crystal. Kumada's invention is not directed to the problem of mitigating beam divergence or correcting unwanted depolarization in coherent light output. In U.S. Pat. No. 3,780,296, Waksberg, et al., disclose apparatus for an electro-optical laser beam modulation system. This device requires a specially birefringent optical medium and employs analyzer and photodetector means for generating and processing an error signal in order to modify the characteristics of a laser beam. This device not only requires a medium which must exhibit particular birefringent behavior, but also necessitates the inclusion of complex electronic control and analysis equipment. U.S. Pat. No. 3,892,469--Lotspeich reveals an apparatus which employs an array of cylindrical electrodes which are embedded in a solid body of crystalline material and are energized by a remote power supply in order to provide a device having a variable focal length. Lotspeich's device requires the precise and costly implantation of metal electrodes in a specialized compensation medium. Additionally, Lotspeich makes no attempt to confront the difficulties imposed by stress-related depolarization of the beam.
None of these prior devices provides an effective and inexpensive solution to the optical aberrations described above in detail which plague the operation and performance of high-onput solid-state lasers. Such a solution would satisfy a long felt need manifested by the current efforts of the laser and optics industry which continues to develop communications and measurement systems which require reliable, durable, and cost-effective high-output lasers. The continued development and manufacture of such high-power lasers has generated a concomitant demand for an invention which compensates for laser beam imperfections in a manner which does not create additional deleterious side-effects and which does not interfere with the performance and amplification of the laser itself.
Such a compensator would ideally be suited to operate inside a repetitively pumped laser resonator and would be required to function effectively over all repetition rates and over a wide range of temperatures which would include operational ranges for military as well as commercial applications.